In its more general aspect, this invention relates to a tire for vehicles comprising:
a carcass structure including a central crown portion and two sidewalls ending into a couple of beads for anchoring to a rim of a wheel;
a belt structure, coaxially associated to the carcass structure;
a tread, coaxially extending around the belt structure, comprising a plurality of blocks, located on opposite parts of an equatorial plane of the tire between a longitudinal groove formed astride said equatorial plane and a couple of longitudinal grooves, said blocks being circumferentially spaced by a plurality of first slits extending in a direction substantially perpendicular to the rolling direction of the tire.
More particularly, the invention refers to a tire which is preferably, however not exclusively, used on driving wheels of heavy vehicles adapted to cover long travels in motorways.
In the following description and in the appended claims, the term: xe2x80x9cblockxe2x80x9d is used to indicate a tread portion comprised between consecutive grooves or slits either in the axial or in the circumferential direction.
In the following description and in the appended claims, the terms: xe2x80x9cgroovesxe2x80x9d and xe2x80x9cslitsxe2x80x9d are used to indicate channels formed in the tire tread and having a width greater than and, respectively, equal to or smaller than 1 mm.
In the following description and in the appended claims, furthermore, the terms: xe2x80x9clongitudinalxe2x80x9d and xe2x80x9clongitudinallyxe2x80x9d, are used to indicate quantities measured along the circumferential development of the tire.
Lastly, in the following description and in the appended claims, the terms: xe2x80x9caxialxe2x80x9d and xe2x80x9caxiallyxe2x80x9d are intended to indicate quantities measured along the peripheral surface of the tire in a direction perpendicular to the equatorial plane of the same.
As is known, in the field of tire production, intended or not for use on heavy vehicles, the need has always been felt of reducing as much as possible the rolling resistance of the tire.
It is also known that the difficulty of fully satisfying the above need is essentially related to the difficulty of reducing the tire rolling resistance without substantially affecting its performances in terms of kilometric yield, good traction capacity, stability on wet or snow grounds, and good lateral stability.
Until now all the attempts made in the art aiming at reducing the rolling resistance of the tire have brought about a more or less marked worsening of one or more of the above-identified features.
So, for instance, it was recently proposed to form in the blocks located in an equatorial zone of the tread a plurality of slits extending in a direction substantially perpendicular to the rolling direction of the tire.
In such case, however, the desired reduction in rolling resistance was obtained at the expense of the traction characteristics of the tire, which have undergone a corresponding worsening.
Therefore, the technical problem underlying the present invention is to provide a tire having structural and functional features which allow to reduce the tire rolling resistance while keeping the tire performances at a very satisfactory level in terms of kilometric yield, good traction and stability on wet or snow grounds, and good performances of lateral stability.
According to a first aspect of the invention, this problem is solved by a tire of the type indicated above, which is characterised in that the tread further comprises a plurality of transversal grooves, pitchwise spaced from one another and extending parallel to said first slits, said grooves being provided with respective second slits extending from a bottom portion thereof.
According to a second aspect of the invention, this problem is solved by a tread for vehicle tires, in particular a pre-moulded tread for cold-covering worn tires, comprising a plurality of blocks located in opposite sides of the equatorial plane of a tire between a longitudinal groove formed astride said equatorial plane and a couple of longitudinal grooves, said blocks being axially crossed by a plurality of first slits extending in a direction substantially perpendicular to the rolling direction of the tire, which is characterised in that it further comprises a plurality of transversal grooves, pitchwise spaced from one another and extending parallel to said first slits, said grooves being provided with respective second slits extending from a bottom portion thereof.
According to the invention, the Applicant has found in particular that by forming, in an equatorial zone of the tread, transversal grooves provided with slits downwardly extending therefrom, it is advantageously possible to achieve both adequate traction characteristics and the desired lower rolling resistance, essentially associated to the overall lower mobility of the tread blocks under the tire ground-contacting area due to the mutual contact of adjoining blocks caused by a closure of the slits.
In the following description and in the appended claims, the blocks located in the equatorial zone of the tread will be indicated for the sake of simplicity by the term: xe2x80x9cequatorial blocksxe2x80x9d.
Preferably, the equatorial zone of the tread concerned by the transversal grooves provided with slits in their lower part extends astride the equatorial plane of the tire and up to the middle of the longitudinal grooves for a length comprised between 40% and 70% of the axial development of the tire tread.
Still more preferably, such equatorial zone extends astride the equatorial plane of the tire for a length comprised between 50% and 60% of the axial development of the tread.
Thanks to the presence of the slits, the tread portions in which the equatorial blocks are formed, are so-to-say embedded into one another under the ground-contacting area of the tire, because of the strains which the tread undergoes during tire rolling.
The ensuing stiffness increase reduces the energy dissipation phenomena due both to the mobility of the blocks and to the physical-mechanical characteristics of the rubber composition of the tread, with a reduction of rolling resistance during running.
A further and important advantageous effect attained by the particular structure of the transversal grooves is represented by the achievement of a better wear regularity, which is also associated to some extent to the stiffness increase of the tread equatorial blocks.
Preferably, the tread is circumferentially provided with a couple of longitudinal slits extending in opposite parts of the equatorial plane of the tire, which slits axially separate the equatorial blocks and split the transversal grooves formed therein into two portions proximal and, respectively, distal with respect to the equatorial plane of the tire. A depth of the longitudinal slits is substantially equal to a sum of depths of the transversal grooves and the second slits, and a depth of the first slits is substantially equal to a sum of the depths of the transversal grooves and the second slits.
Advantageously, the longitudinal slits contribute both to increase the tire road holding in the axial direction, whenever the same is stressed in a transversal direction with respect to the rolling direction, and to prevent the triggering of phenomena of irregular wear of the blocks.
Preferably, the longitudinal slits extend along substantially the whole circumferential development of the tire according to an essentially zigzag path, which advantageously enhances the embedding between axially adjoining blocks, with a further reduction in energy dissipation phenomena due to block mobility in the longitudinal direction.
In the tire of the invention, the transversal grooves preferably have a reduced depth compared to the tires of the known art, and such as not to exceed 18 mm together with the slits downwardly extending from their bottom.
Such a reduced depth of the grooves allows to achieve several important advantages.
Firstly, it allows to effectively contribute to reduce the rolling resistance of the tire, thanks to the reduction in the volume of the rubber composition required to form the tread and, along therewith, to reduce the hysteretic energy dissipation phenomena induced during rolling.
Secondly, it allows to achieve both a good wear regularity with a substantial absence of the so-called xe2x80x9cmillingxe2x80x9d wear on the block outlet edge, obtained thanks to the limited block flexibility, and a good noiselessness, obtained thanks to the reduction of the equatorial blocks vibration phenomena and to the reduction of the so-called xe2x80x9cair pumpingxe2x80x9d acoustic phenomenon generated by the cyclic compression of the air trapped in the transversal grooves.
In a preferred embodiment, the transversal grooves have a depth ranging between 1 and 4 mm in the portion proximal to the equatorial plane of the tire and a depth ranging between 5 and 10 mm in the portion distal to said equatorial plane.
In this way, an optimal balance is achieved between the traction characteristics and the wear resistance characteristics of the tire tread.
Preferably, the sum of the depths of the transversal grooves and of the slits downwardly extending therefrom has a substantially constant value along the axial development of the same and is comprised between 13 and 18 mm.
In a preferred embodiment of the invention, in the portion of the transversal grooves proximal to the equatorial plane of the tire, the slits downwardly extending from the bottom of the grooves have a depth comprised between 60% and 90% of the sum of the depths of the transversal grooves and said slits.
On the contrary, in the portion of the transversal grooves distal with respect to the equatorial plane of the tire, the slits downwardly extending from the bottom of the grooves have a depth of between 40% and 70% of the sum of the depths of the transversal grooves and said slits.
Thanks to this feature, an optimal embedding effect is ensured between adjoining blocks, whatever the block wear condition is.
In a preferred embodiment, both the transversal grooves and the slits downwardly extending from their bottom extend substantially along the entire axial development of the blocks and have at least a substantially zigzag-shaped portion.
Preferably, the substantially zigzag-shaped portion is located in the portion of transversal grooves and of the slits distal with respect to the equatorial plane of the tire.
In this way, the embedding between tread portions concerned by adjoining equatorial blocks is promoted, with a further reduction of the energy dissipation phenomena due to the mobility of the equatorial blocks and, along therewith, of the tire rolling resistance.
Alternatively, such embedding effect between tread portions concerned by adjoining equatorial blocks can be promoted by forming substantially as a cuspxe2x80x94i.e. by causing an abrupt inclination change with respect to its axial developmentxe2x80x94at least a portion of the transversal grooves and of the slits downwardly extending therefrom.
Preferably, the substantially cusp-shaped portion is located in the portion of the transversal grooves and of the slits proximal to the equatorial plane of the tire.
According to a preferred embodiment of the invention, the path of the transversal grooves and of the related slits forms a segmented line wherein at least a substantially zigzag-shaped portion and a substantially cusp-shaped portion are both present.
Advantageously, in this way said embedding effect between tread portions concerned by adjoining equatorial blocks is optimised, and the maximum effect of reduction of the tire rolling resistance is achieved.
Preferably, moreover, the transversal grooves and the slits downwardly extending therefrom confer a directional characteristics to the tread pattern, i.e. they impart a preferred rolling direction to the tire.
In any case, the invention may also be implemented according to non-directional (symmetrical or asymmetrical) tread patterns that lack such preferred rolling direction.
Advantageously, the transversal grooves have a width that increases moving away from the equatorial plane of the tire, so as to enhance the tire capacity of draining off the water present under the tire ground-contacting area and reduce the aquaplaning phenomena.
In order to help a progressive entry of the transversal grooves under the ground-contacting area of the tire and to reduce as much as possible its noisiness during running, the transversal grooves advantageously form an angle (xcex1) of from 10xc2x0 to 25xc2x0 with respect to a plane perpendicular to the equatorial plane of the tire.
Preferably, at least one of the blocks formed in the equatorial zone of the tread has a beveled corner at a longitudinal groove formed astride the equatorial plane of the tire and/or at the longitudinal grooves circumferentially extending in a direction substantially parallel to the tire rolling direction.
In this way, the number of edges that may trigger phenomena of irregular wear, due to their mobility, is advantageously adequately reduced.
In a further preferred embodiment of the invention, the longitudinal grooves exbend substantially along the whole circumferential development of the tire, according to a substantially zigzag-shaped path.
Advantageously, in this way the traction capacity of the tread in the tire rolling direction is further increased.
Preferably, the tire of the invention further comprises a plurality of lateral blocks, having an outer surface provided with facets, formed in opposite side zones of the tread external to said equatorial zone.
Advantageously, the outer faceted surface of said lateral blocks effectively contributes to reach the desired traction characteristics of the tread in the tire rolling direction.
Preferably, at least one of said lateral blocks has a beveled corner at the longitudinal grooves circumferentially extending in a direction substantially parallel to the tire rolling direction and/or at its own outer faceted surface.
In this way, the number of points suitable to trigger an irregular wear of the tire is adequately reduced.
In order to further increase the kilometric yield, the tire of the invention preferably has a high solid/total volume ratio under its ground-contacting area.
In other words, the overall volume taken up by the blocks (equatorial and lateral) i.e. the volume of the solids in a tread portion having a length equal to the pitch of the tread pattern and a width equal to the tread axial development, is preferably comprised between 80% and 85% of the overall volume of said tread portion.
In the following description and in the appended claims, the term: xe2x80x9caxial development of the treadxe2x80x9d, is used to indicate the extension in width of the latter as measured along the peripheral surface of the tire.
In the following description and in the appended claims, the term: xe2x80x9cpitch of the tread patternxe2x80x9d is used to indicate the length, measured along the circumferential development of the tread, of a portion of the tread pattern which periodically repeats for a finite number xe2x80x9cnxe2x80x9d of times throughout the whole circumferential development of the tread.
In the present case, therefore, the pitch of the tread pattern is equal to the distance between the starting points of two subsequent repetitive portions of the tread pattern, measured along the circumferential development of the tread.
Preferably, in the equatorial zone of the tread the volume of the solids, i.e. the volume taken up by the equatorial blocks, is preferably comprised between 80% and 90% of the overall volume of said tread portion, while in the lateral zones the volume taken up by the lateral blocks is slightly lower, in order to improve the traction characteristics and is comprised between 75% and 85% of the overall volume of said tread portion.
The tire and the tread of the invention may be manufactured by means of a process comprising a plurality of production steps which are quite conventional per se and known in the art.
More particularly, such process comprises the steps of preliminarly and independently preparing several semi-finished products corresponding to the different parts of the tire (carcass plies, belt strips, bead wires, fillings, sidewalls and treads), which are successively assembled to one another by means of a suitable assembling machine.
The subsequent vulcanisation step then welds together the above semi-finished products to form an integral block, i.e., the tire.
Clearly, the step of preparing the above semi-finished products is preceded by a step of preparing and forming the corresponding rubber compositions, which comprise at least an unsaturated chain cross-linkable polymer base.
For the purposes of the invention, suitable polymer bases are those selected from among the group comprising: natural rubber, 1,4-cis polybutadiene, polychloroprene, 1,4-cis polyisoprene, optionally halogenated isoprene-isobutene copolymers, either prepared in solution or in emulsion, ethylene-propylene-diene terpolymers.
According to a preferred embodiment of the invention and in order to obtain an optimal kilometric yield, a good grip on wet road and an optimal tear resistance of the tread, the aforementioned polymer bases are used in the rubber composition in such quantities and proportions as to achieve a tread abradabilityxe2x80x94determined according to Standards DIN 53516xe2x80x94not exceeding 70 mm3.
The determination of the optimal makeup of the rubber composition and its preparation by means of conventional mixing operations, may be easily performed by those skilled in the art.
A typical rubber composition suitable to achieve the optimal abradibility characteristics (in parts by weight) is given hereunder solely by way of non limitative example: